The present invention generally relates to a design for and a method of making a sputter target assembly for physical vapor deposition thin film coating processes. In particular, the invention relates to a sputter target assembly having a cobalt target diffusion-bonded to an aluminum or copper backing plate, and a method for making such assemblies.
Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates. Basically, a cathode assembly including the sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits on the receiving surface of the substrate to form a thin layer or film.
The sputter target is heated during the sputtering process by the kinetic energy of the bombarding gas ions imparted to the sputtering target as thermal energy. This heat is dissipated by thermal exchange with a cooling fluid typically circulated beneath or around a backing plate which is bonded to the sputter target along an interface opposite the sputtering surface.
Sputter targets of high-purity metals or metal alloys attached to aluminum- or copper-based backing plates are typically used to deposit thin films on substrates such as, for example, semiconductor devices. These sputter target assemblies provide mechanical and electrical attachment of the target material to the sputter apparatus, provide vacuum sealing surfaces to maintain proper sputter chamber environmental conditions, and typically provide a path of heat removal for effective cooling of the target material during sputter deposition.
High-purity metal and metal alloy sputter targets historically have been solder-bonded to copper alloy backing plates. In particular, solder bonding is currently used to bond cobalt sputter targets to copper or aluminum backing plates. However, as sputter target assemblies undergo heating and cooling, the bond between the target and the backing plate is subjected to stresses that may lead to failure of the bond. In particular, solder bonding typically results in relatively weak bond strength between cobalt sputter targets and copper or aluminum backing plates. The bond strength further decreases as temperature increases to those typically encountered during the sputtering process. Higher melting temperature solders used for high power applications are stronger but are less forgiving of the stresses developed in the material systems. In addition, typical solders have low melting points and high vapor pressures and represent a potential source of contamination within the sputtering chamber during sputter deposition. Currently, solder bonding processes produce bonds having approximately 3 ksi (2.times.10.sup.7 N/m.sup.2) bond strength for cobalt targets bonded to copper backing plates. Sputter target assemblies comprising cobalt targets bonded to aluminum backing plates exhibit even weaker bond strength. As a consequence, the use of solder bonding has imposed practical limits on the sizes of sputter targets and the levels of sputtering power which sputter target assemblies can sustain.
Accordingly, there remains a need for a method of bonding high-purity cobalt and cobalt alloys to copper and aluminum backing plates so that the bonded joint will be mechanically sound so as to allow for higher levels of sputtering power and larger sputter target designs without risk of deflection or debonding in use at elevated temperatures.
Additionally, there is a need for sputter target assemblies having cobalt targets and copper or aluminum backing plates that can withstand prolonged high-power sputter usage periods without significant target-backing plate debonding.